Pulsed Plasma Thrusters (PPT) are used to provide periodic pulses of thrust for satellites in space. Prior art high voltage PPTs were constructed from coaxial electrodes with a PTFE propellant in a coaxial configuration such as U.S. Pat. No. 6,269,629 by Spanjers, and U.S. Pat. No. 6,295,804 by Burton et al, or in a parallel plate configuration such as U.S. Pat. No. 6,373,023 by Hoskins et al. These prior art PPTs are ignited and driven with high voltages stored in capacitors, with or without an external spark gap initiator. The energy storage of a capacitor may be expressed as (½)CV2. Charging of the storage capacitors may be accomplished using high voltage supplies or by low voltage supplies followed by DC-to-DC converters which convert a low voltage into the necessary high voltage to charge the storage capacitor. The voltage stored in the capacitor results in a plasma discharge across the surface of an insulator made from a material such as PTFE (also known as Teflon®), which results in thermionic surface heating of the PTFE, and high speed discharge of the superheated PTFE particles and related plasma-PTFE byproducts. The superheated PTFE accelerates through an exit aperture, producing a reactive force for pulsed thrust control. Another prior art low voltage PPT uses a conductive propellant such as carbon whereby the ohmic heat generates a surface plasma, which releases particles of superheated carbon at high speed, as described in U.S. Pat. No. 6,153,976 by Spanjers. The previous examples of prior art used capacitors as a source of energy storage. Attempts to drive plasma sources with inductors have been made in the past but were abandoned due to the need for very high voltages to break-down the vacuum gap and the associated requirement that the electronic switch controlling the inductor must operate very fast and hold-off said high voltage. In the field of plasma assisted physical vapor deposition, a new plasma initiation method was introduced that employed surface breakdown along a metallized insulator separating anode and cathode to reduce the initiation voltage, as described in U.S. Pat. No. 6,465,793 by Anders. This reference describes a capacitive driver and a pulse-forming network which is charged up to a voltage allowing the surface breakdown to occur, typically in excess of 1000V. The storage capacitor is charged by a voltage supply providing the required 1000V. Inductive energy storage ignition has been used in the past but was not used in connection with the above mentioned low voltage initiation and therefore required the output of very high breakdown voltages, which had to be held off by some kind of switching device making this approach very complicated due to the lack of adequate compact semiconductor devices. The prior art systems using either a storage capacitor charged to a high voltage or inductive energy storage required high speed switching of large voltages, which is difficult to do without incurring switching losses, and also typically restricts or eliminates the use of semiconductor devices because of the high voltage requirements. In addition, the use of capacitors adds a significant amount of mass to the systems and limits the lifetime as high voltage capacitors have been shown to deteriorate with time.
A new class of device is known as a vacuum arc thruster (VAT), which contrasts with the prior art Pulsed Plasma Thruster (PPT) in several ways. The prior art PPT uses a surface discharge, which ablates the insulator material as a propellant, and avoids eroding the electrodes. The acceleration mechanism of the PPT is dominated by a j×B force. The vacuum arc thruster (VAT) uses the cathode material as the propellant, which forms a low impedance plasma. The acceleration mechanism is dominated by pressure gradients formed by the expanding plasma, in addition to the j×B force described earlier. The ignition mechanism is also different between a PPT and a VAT. The VAT uses a voltage breakdown across a very small gap, while the PPT uses a surface discharge, which is frequently assisted by a spark plug or even a laser. References to the present invention will refer to a vacuum arc thruster (VAT) to contrast from the prior art pulsed plasma thruster (PPT). In the present invention, the electrodes are the propellant and the insulator is not consumed by the plasma. The voltage and current characteristics through the plasma discharge are different between the present VAT invention and the prior art PPT. After ignition, the VAT operates for the rest of the pulses at a fairly constant voltage and the current reduces, whereas the voltage and current characteristics of a PPT are the opposite.
What is desired in a VAT is a low mass, low voltage device (<1000V) which uses inductive energy storage rather than capacitive energy storage, which forms a plasma from a conductive layer of material which is formed over an insulator surface, where the conduction layer is a different or the same type of material as used in the cathode, and which provides an electrode geometry which is either parallel plate or coaxial.